This invention relates to crossed-field amplifiers in which the signal to be amplified is provided to the slow-wave structure of the crossed-field amplifier and the amplified signal is obtained by coupling to the slow-wave structure after amplification of the RF field in the interaction space has occurred.
The conventional prior art crossed-field amplifier 10 of FIG. 1 is an efficient high power broadband power amplifier. The gain is low so that relatively high drive power from frequency source 16 is necessary to achieve stable input/output lock operation. In the conventional crossed-field amplifier 10, the RF drive signal is introduced at the input 12 of the anode slow-wave structure 11 and the RF output power is collected in load 14 at the output 13 of the anode as schematically shown in FIG. 1. In this amplifier, a cathode 15 is a smooth cylinder on which a secondary emitter material has been placed at least in the region of the cathode radially opposite to the circumferential extent of the slow-wave structure of the anode. In the conventional crossed-field amplifier 10, the electron cloud at the cathode does not have a strong frequency determining component because of the weak incident drive signal at the cathode 15 provided by the field originating at the anode slow-wave structure 11. The conventional tube, therefore, produces noise which is typically at a level of -50 db per MHz below the level of the output signal in the load 14.
In order to reduce the input drive signal power level, the prior art cathode driven crossed-field amplifier tube 27 of FIG. 2 was developed. The tube 27 is shown schematically as having a cathode slow-wave structure 21 and an anode slow-wave structure 22. The cathode slow-wave structure was built as an integral part of the cathode and had matching dispersion characteristics with the slow-wave structure 22 of the anode. The input signal is applied by the frequency drive source 23 to one end of the cathode slow-wave structure 21 which was terminated at its other end in a matched termination 24. The anode slow-wave structure 22 was terminated at one end in a matched termination 25 and at its other end was connected to a load 26 which preferably was also a matched load. With the tube 27 schematically shown in FIG. 2, comparable power outputs to that of the tube of FIG. 1 were achievable with an order of magnitude weaker drive signal. However, the cathode driven crossed-field amplifier of FIG. 2 produced noise which was comparable with the crossed-field amplifiers of FIG. 1; namely, a signal-to-noise ratio in the order of 50 db per megahertz.